Elevated CO2 magnifies the effect of eutrophication and shifts phytoplankton community composition

Background/Question/Methods

We are currently experiencing a dramatic and continued increase in atmospheric CO₂ concentration that leads to higher aqueous CO₂ concentrations and necessitates a re-evaluation of the role of carbon (CO₂) relative to other nutrients in freshwater systems. These changes in atmospheric CO₂ concentration are occurring at the same time as increases in the availability of phosphorus and nitrogen in ecosystems throughout the world.  These changes are bound to affect phytoplankton, a key element of the global carbon cycle and the base of most aquatic food webs.  We performed lake mesocosm experiments designed to investigate the effect of elevated CO₂ and eutrophication on phytoplankton productivity and community composition, with a particular interest in their interaction.  Our experiment was designed to explore the extremes of the range of nutrient and CO₂ availability, with high concentrations of nutrients and CO₂that provided nutrient-replete conditions, similar to nutrient-rich media in the laboratory. Our study was conducted across different periods of a growing season to account for seasonal differences in physical and biotic conditions.

Results/Conclusions

We found that elevated CO₂ weakly stimulated total phytoplankton growth in the absence of fertilizer.  There were pronounced changes in community composition, however: in particular, chlorophytes increased in frequency at elevated CO₂ independent of nutrient status.   This corroborates our laboratory findings, which indicated that phytoplankton community composition could be influenced by CO₂ concentration in a predictable way.  Furthermore, we found that CO₂ greatly magnified the increase in phytoplankton standing stock caused by fertilization.  These changes may have substantial impacts on nutrient and CO₂ cycling and may alter whole freshwater ecosystems through bottom-up trophic effects.  Furthermore, these findings may highlight the relation between the rate of provision of a nutrient, either as an inexhaustible flux or an exhaustible pulse, and their importance in limiting growth rate and yield respectively may be generalizable beyond phosphorus, nitrogen and CO₂.  This research is a first step towards predicting the combined effect of CO₂ and eutrophication on whole communities in freshwater systems, from plankton to fish, and the potential changes in the role of these systems in global geo-chemical cycles.